This invention relates to papermaking and more particularly to an apparatus for controlling the temperature of the sheet through the controlled application of steam against the sheet surface.
In the mechanical pressing of paper the drainage rate is proportional to the viscosity and surface tension of the trapped water. The increasing of sheet temperature decreases the water viscosity and surface tension, hence augmenting the pressing process. As shown in U.S. Pat. Nos. 3,574,338; 3,945,570; 4,050,630 and 4,163,688, it is common practice to apply steam to a sheet immediately prior to the sheet entering the press so that the latent heat of the steam heats the sheet.
The press is located before the dryer section of a paper-machine. Therefore, increasing the water removal rate through the press serves to decrease the sheet moisture content of a sheet entering the dryer-section, thereby either reducing the energy consumption required to further dry the sheet or increasing production (speed) at constant dryer section energy consumption.
The controlled application of steam at equally spaced increments across the machine can be employed to control the initial and hence final moisture profile of the sheet. At the dry end of the machine, following the dryer-section, the sheet is passed through a vertical column of horizontal-axis rolls known as a calender-stack. The surface finish and thickness or caliper of the sheet is directly affected by both the contact pressure between two adjacent rolls of the stack through which the sheet passes and the compressibility and shear modulus of the sheet, which are proportional to the moisture and temperature profile of the sheet (although not exclusively). Applying steam to the sheet using "steam-showers" will affect both the moisture and temperature of the sheet, and hence, the caliper and surface finish qualities (such as gloss) of the sheet. Applying a uniform amount of steam across the machine can thus decrease the caliper of the sheet leaving the calender stack and increase the gloss of the final product. Applying controlled amounts of steam at selected positions across the machine can thus be used, by extension, to control the caliper and/or gloss profile of the final product.
In any steam application, the effective usage of the consumed steam should be maximized. To maximize the effective usage, the percentage of consumed steam that condenses on the sheet for the purpose of raising the sheet temperature should be maximized, and the percentage of consumed steam that does not condense which exhausts to the atmosphere as wasted energy should be minimized.
There are certain applications where the steam application does not have to be positionally and volumetrically controlled. In other applications, however, it is necessary to impart steam to the process in controlled amounts at specified positions across the machine for profiling certain sheet qualities. This controlled imparting of steam is commonly performed as part of a closed-loop control system, where the sheet quality variable in question is scanned on-line at equally spaced increments across the machine, and the results obtained by the scanning device, through the use of computer analysis, are used to automatically control the steam-flow applied to the sheet in accordance with the desired sheet quality criteria.
The ability of known steam-shower apparatus to repeatedly apply a uniform steam flow is presently limited to the accuracy and repeatability of pneumatically actuated control valves, whose pneumatic-supply signal must first be converted from a computer level electric signal to a pneumatic counterpart using a current to pressure transducer, the combined accuracy and repeatability of which is questionable.
For the same reasons that it is important to control accurately the steam-flow to the application, it is also important to maintain uniform heat-transfer, over the portion of the sheet in question.
It is therefore a principal object of the present invention to provide a steam-shower apparatus which maximizes the percentage of consumed steam that condenses on the sheet.
Another object of the present invention is to provide a steam-shower apparatus which applies steam in such a way that the entrainment of non-condensable air into the condensation space, which severely hampers condensation heat transfer, is limited or eliminated.
It is another object of the present invention to provide a steam-shower apparatus which insures that the high initial relative velocities between the steam and the sheet, as required to provide effective heat transfer, are maintained over the full machine-direction length of the application apparatus.
Still another object of the present invention is to provide a steam-shower apparatus which insures that the steam-flow is made to travel over the full length of the steam application apparatus so that the time of contact between each volumetric unit of steam and the sheet is maximized.
Yet another object of the present invention is to provide a steam-shower apparatus that accurately and repeatably applies steam to a sheet at any required position across the machine.
Another object of the present invention is to provide a steam-shower apparatus that allows for improved accuracy and repeatability of steam-flow control.
Still another object of the present invention is to provide a steam-shower apparatus that applies steam to a sheet in such a way that uniformity of heat-transfer is provided in the cross-machine direction.
A further object of the present invention is to provide a steam-shower apparatus that applies the steam in such a way that the steam does not impinge on the sheet directly.